materials Article
Effects of the Tempering and High-Pressure Torsion Temperatures on Microstructure of Ferritic/Martensitic Steel Grade 91 Artur Ganeev 1, *, Marina Nikitina 1 , Vil Sitdikov 1 , Rinat Islamgaliev 1 , Andrew Hoffman 2 and Haiming Wen 2,3 1 2 3
*
Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, Ufa 450008, Russia;
[email protected] (M.N.);
[email protected] (V.S.);
[email protected] (R.I.) Department of Mining and Nuclear Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA;
[email protected] (A.H.);
[email protected] (H.W.) Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA Correspondence:
[email protected]; Tel.: +7-347-273-5244
Received: 22 March 2018; Accepted: 16 April 2018; Published: 19 April 2018
Abstract: Grade 91 (9Cr-1Mo) steel was subjected to various heat treatments and then to high-pressure torsion (HPT) at different temperatures. Its microstructure was studied using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Effects of the tempering temperature and the HPT temperature on the microstructural features and microhardness in the ultrafine-grained (UFG) Grade 91 steel were researched. The study of the UFG structure formation takes into account two different microstructures observed: before HPT in both samples containing martensite and in fully ferritic samples. Keywords: ferritic/martensitic steel; high-pressure torsion; ultrafine-grained structure
1. Introduction Recently, much attention has been paid to research into the microstructure and properties of ferritic/martensitic steels and their use at elevate temperatures [1–5]. In particular, various thermal and thermomechanical treatments were applied to Grade 91 steel to study changes in microstructure [1,2]. For example, the thermomechanical treatment proposed in [5], where the steel was deformed in the austenitic regime, was aimed at refining the austenite grains and enhancing the effect of impeding dislocations by carbonitrides [3]. Warm rolling was used to increase the density of dislocations, grain boundaries, and subgrains as the nucleation sites of stable fine particles. It was demonstrated that main microstructural features which contribute to the enhancement in strength are reduced width of martensitic laths, and the presence of thermally stable precipitates [4,5]. At the same time, the strength of martensitic steels can be enhanced by grain refinement, using severe plastic deformation (SPD) [6,7], which is based on the application of high strains at low homologous temperatures. For example, equal-channel angular pressing (ECAP) allows the production of ultrafine-grained (UFG) structure in ferritic/martensitic steels, which increases their strength [8]. Therefore, research into the effect of grain refinement on strength of ferritic/martensitic steels are of special interest. For example, in [9–11], the formation of an ultrafine microstructure was attributed to the fine martensite starting microstructure. At the same time, it is recognized that the most significant grain refinement can be achieved by applying high-pressure torsion (HPT) [12]. The grain refinement to 130 nm in ferritic/martensitic steels produced by HPT was observed in [13,14]. Application of HPT to transformation induced plasticity (TRIP) steel leads to a gradual reverse transformation Materials 2018, 11, 627; doi:10.3390/ma11040627
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from body centre cubic (bcc) α’ phase to the hexagonal close-packed (hcp) ε phase [15]. A strong grain refinement at the HPT treatment was also noticed in austenitic stainless steels [16–20], which was accompanied by a deformation-induced martensitic transformation. However, the effect of pre-processing microstructure (purely ferrite or partially containing martensite) on microstructure and properties of UFG ferritic/martensitic steels processed by HPT has not been study. Therefore, the main focus of this work was on the investigation of the impact of pre-processing microstructure on post-processed microstructure of ferritic/martensitic steel Grade 91 subjected to HPT. 2. Materials and Methods Hot-rolled steel Grade 91 rods, whose chemical composition is displayed in Table 1, were used as starting material for investigations. The as-received rods were heated in the austenitic regime at 1050 ◦ C for 1 h and quenched in oil. Then, the quenched samples were tempered at two different temperatures above and below the martensitic transformation temperature. It is known that the martensitic transformation temperature in steels with similar chemical composition starts from approximately 600 ◦ C [21]. Therefore, two tempering temperatures were chosen in this work: (1) tempering at 800 ◦ C leading to complete formation of ferrite, henceforth this treatment is referred to as T800, and (2) tempering at 500 ◦ C where the retention of residual martensite was expected, henceforth this treatment is referred to as T500. Table 1. Chemical composition of steel Grade 91, wt %. C
Mn
P
S
Cu
Si
Ni
Cr
Mo
V
Ti
N
0.08
0.53
0.016
0.003
0.09
0.28
0.13
8.43
0.9
0.225
0.01
0.038
After the tempering, discs with the initial thickness of 1.5 mm were cut out of rods and subjected to HPT. During HPT, the shear strain γ along the radius r is given by γ = 2πrn/h where h is the thickness of the disk and n is the number of rotations. It should be noted that in recent publications it was shown that this equation is true only for a certain condition [22,23]. The samples were deformed under the pressure of 6 GPa for 10 rotations, with a rotation rate of 0.2 rpm at 20 and 300 ◦ C. The Vickers microhardness (HV) measurements were performed with a microhardness tester from Buehler (Micromet 5101, Uzwil, Switzerland) with a load of 100 g and a dwell time of 15 s. Each measurement was carried out at a distance Rn to the disk edge. For each sample, such measurements were made by at least 6 lines. The microstructure was studied using a transmission electron microscope (TEM) JEOL JEM 2100 (JEOL Ltd., Tokyo, Japan). The samples for TEM were cut from half of radius of the HPT disc, and thinned to electron transparency by jet electropolishing in a solution of perchloric acid in butyl alcohol with a voltage of 56 V. The grain size was estimated from TEM on the bright and dark field images, where grain boundaries are clearly visible. The grain size was estimated with the grain-by-grain measurement method, using manual measurement of approximate minimum and maximum diameter of each grain. Average values were obtained from 200–300 measurements. X-ray diffraction (XRD) was performed using a DRON-4M diffractometer (Bourevestnik Inc., S-Peterburg, Russia) with CoKα radiation (35 kV, 30 mA). The reflected beam was filtered using graphite monochromator. The lattice parameter, size of coherent scattering domains (CSD), and dislocation density were estimated using the whole pattern approach, which was implemented in the PM2K software (version 2.10) [24]. The instrumental broadening of diffraction lines, i.e. the parameters U, V, W, a, b, and c for the Cagliotti function, were determined by processing a diffraction pattern from LaB6, which was obtained in the same conditions as the studied samples.
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3. Results 3. Results Figure 1 displays TEM images showing the microstructure of the Grade 91 steel after Figure 1 displays TEM images showing the microstructure of the Grade 91 steel after tempering at tempering at two different temperatures. Tempering at 500 °С leads to formation of martensite laths two different temperatures. Tempering at 500 ◦ C leads to formation of martensite laths with an average with an average width of ~200 nm at the boundaries of prior austenite grains. After tempering at ◦ C, the average width of°С, ~200 at thegrain boundaries of µm. priorInaustenite After tempering at in 800 800 thenm average size is ~0.5 addition, grains. carbide precipitates ~0.2 µm diameter are graindetected. size is ~0.5 µm. In addition, carbide precipitates ~0.2 µm in diameter are detected. Materials 2018, 11, x FOR PEER REVIEW
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3. Results Figure 1 displays TEM images showing the microstructure of the Grade 91 steel after tempering at two different temperatures. Tempering at 500 °С leads to formation of martensite laths with an average width of ~200 nm at the boundaries of prior austenite grains. After tempering at 800 °С, the average grain size is ~0.5 µm. In addition, carbide precipitates ~0.2 µm in diameter are detected.
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(b)
Figure 1. Bright-field transmission electron microscopy (TEM) images of Grade 91 microstructure
Figure 1. Bright-field transmission electron microscopy (TEM) images of Grade 91 microstructure after after quenching and tempering at: (a) 500 °C; (b) 800 °C quenching and tempering at: (a) 500 ◦ C; (b) 800 ◦ C Figure 2 shows the microstructure of Grade 91 steel after HPT. HPT at room temperature Figure 2inshows the grain microstructure Grade 91 steel after HPT. at room temperature resulted resulted dramatic refinement.of Grains with an average lengthHPT of ~250 nm and width of ~100 nm which were stretched along initial martensite laths,ofare formed in samples T500 in dramatic grain refinement. Grainsthe with an average length ~250 nm and width ofafter ~100 nm +which (Figure 2a).the Equiaxed grains with laths, an average diameter ~250 nm are T500 observed after T800 + 2a). wereHPT20 stretched along initial martensite are formed in of samples after + HPT20 (Figure HPT20 (Figure 2d). The formation of elongated grains along the deformation direction at lower Equiaxed grains with an (a)average diameter of ~250 nm are observed after (b) T800 + HPT20 (Figure 2d). magnification (Figure 2b) can be seen.
The formation of elongated grains along the deformation direction at lower magnification (Figure 2b) Figure 1. Bright-field transmission electron microscopy (TEM) images of Grade 91 microstructure can be after seen.quenching and tempering at: (a) 500 °C; (b) 800 °C It is known that an increase in HPT temperature activates the processes of dislocation climb and Figurethe 2 shows the of microstructure Grade 91 steel after HPT. HPT atThis roommakes temperature accelerates diffusion carbon and of other alloying elements in steels. it possible to resulted dramatic refinement. Grains with an and average length of ~250homogeneous nm and width distribution of ~100 reduce theindensity of grain defects in the microstructure, achieve a more of nm which were stretched along the initial martensite laths, are formed in samples after T500 + carbides. The grain length along the martensite laths is ~450 nm, and the width is ~200 nm in steel HPT20 (Figure 2a). Equiaxed grains with an average diameter of ~250 nm are observed after T800 + after T500 + HPT300 treatment (Figure 3a). Equiaxed grains with an average diameter of ~350 nm are HPT20 (Figure 2d). The formation of elongated grains along the deformation direction at lower formed in steel after T800 + HPT300 (Figure 3b). magnification (Figure 2b) can be seen.
(a)
(b)
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(b)
Figure 2. Cont.
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(c)
(d)
(e)
(f)
Figure 2. Bright-field TEM images of Grade 91 microstructure: (a,c) after tempering at 500 and HPT
Figure 2. Bright-field TEM images of Grade 91 microstructure: (a,c) after tempering at 500 and HPT at 20 °С, where the arrows show the initial boundaries of martensite laths; (b,d) after tempering at at 20 ◦ C, where the arrows show the initial boundaries of martensite laths; (b,d) after tempering 800 and HPT at 20 °С; dark-field TEM images of the Grade 91 microstructure; (e) TEM after T500 + at 800 and HPT at 20 ◦ C; dark-field TEM images of the Grade 91 microstructure; (e) TEM after HPT300 where the arrows show carbides M23C6; (f) after T800 + HPT200 where the arrows show MX T500 + HPT300 where the arrows show carbides M23 C6 ; (f) after T800 + HPT200 where the arrows carbonitrides. show MX carbonitrides.
It is known that an increase in HPT temperature activates the processes of dislocation climb andPlate accelerates the diffusion of carbon othernm alloying in in steels. makes it possible precipitates of carbides up and to ~200 long elements were seen the This initial samples after T500 to reduce the density of defects in the microstructure, and achieve a more homogeneous treatment (Figure 4a). After T500 + HPT20, additional precipitation of carbides with an average distribution of carbides. Theobserved grain length the4b. martensite laths in is the ~450HPT nm, temperature and the width diameter of ~10–30 nm was too,along Figure The increase atisT500 + ~200 nm in steel after T500 + HPT300 treatment (Figure 3a). Equiaxed grains with an average HPT300 treatment resulted in some spheroidization of carbides and an increase in their sizes up to diameter of ~350 nm are formed in steel after T800 + HPT300 (Figure 3b).
~50 nm, Figure 4c. After the T800 treatment, coarse particles of carbides of ~100–300 nm were observed in the microstructure. The carbides were located mainly at grain boundaries, and dispersed particles of fine secondary MX (M—V, Nb, Ti, Mo; X—C, N) carbonitrides with an average size of ~50 nm were found at grain boundaries and at prior austenite boundaries (Figure 3b). The T800 + HPT20 treatment resulted in additional precipitation of carbonitrides from the solid solution, and the particles with sizes of ~5–10 nm precipitated mainly inside grains (Figure 5b). The sizes of coarse carbides were not changed during HPT, and other carbides with sizes
